1. Technical Field of the Invention
The present invention relates generally to spark plugs for use in internal combustion engines of automotive vehicles and cogeneration systems.
More particularly, the invention relates to a spark plug for an internal combustion engine which has a capability to reliably induce and detect an ion current within a combustion chamber of the engine, and a method of manufacturing such a spark plug.
2. Description of the Related Art
FIG. 12 shows an existing spark plug 9 for use in an internal combustion engine, for example, of an automotive vehicle. As shown in the figure, the spark plug 9 includes an insulator 92, a cylindrical center electrode 93, a tubular metal shell 94, and a pair of ground electrodes 95.
The center electrode 93 is secured in the insulator 92 and has a protruding portion 932 that protrudes from an end 921 of the insulator 92. The insulator 92 is retained in the metal shell 94 such that the end 921 thereof protrudes from an end 941 of the metal shell 94. Each of the ground electrodes 95 has a base end 952, which is joined to the end 941 of the metal shell 94 by, for example, welding, and a tip end 951 that faces the side surface of an end portion 931 of the protruding portion 932 of the center electrode 93 through a spark gap 91 in the radial direction of the center electrode 93.
Such a spark plug 9 is generally used to ignite the air/fuel mixture within a combustion chamber of the internal combustion engine. In addition, the spark plug 9 may also be used, as disclosed in Japanese Patent First Publications No. 2000-34969 and No. 2004-22450, to induce and detect an ion current within the combustion chamber of the engine for the purpose of increasing the output and improving the fuel economy of the engine.
Specifically, during combustion of the air/fuel mixture within the combustion chamber of the engine, positive and negative ions are created due to ionization of the air/fuel mixture. The positive and negative ions are absorbed by the corresponding electrodes of the spark plug 9, thereby inducing the ion current that flows between the center electrode 93 and the ground electrodes 95. Through detecting the ion current, it is possible to determine the combustion pressure, the occurrence of a misfire, and other parameters and events relating to the combustion.
However, with running of the engine, the surface of the protruding portion 932 of the center electrode 93 is gradually fouled with combustion residues, such as carbon, thereby reducing the “effective area” of the surface. The effective area of the surface here denotes an area of the surface which is not fouled with combustion residues and thus can absorb ions to induce the ion current.
To enable the effective area of the surface of the protruding portion 932 of the center electrode 93 to be sufficiently large, the Japanese Patent First Publication No. 2004-22450 discloses a spark plug, in which the end face area of the tip end 951 of each of the ground electrodes 95 is specified to be in a given range, so as to effectively burn-off, by spark discharges, the combustion residues that have deposited on the surface of the protruding portion 932.
However, with such a spark plug, it is still difficult to effectively burn-off in a sufficiently wide range the combustion residues, and thus it is still difficult to reliably induce and detect the ion current within the combustion chamber of the engine.